I think you need the silver and gold keys to open gates F and D. (I
never tried to open the doors without the keys <g>)

The silver key can be obtained easily enough by using the switch to
the left and crossing the stone bridge when you first enter the area.

Once you go through the F gate, you'll have to face the first of the
two level bosses - the Massacre Machine (HP 1800). Not too tough a boss
to beat if you have your party full recovered at the save point before
you go in and you use judicious physical attacks by Gadwin along with
lots of magic (provided that you pumped up a character's - for instance
Justin <g> - magic beforehand by making them use magic often ...)

You can get the gold key by exploring the area behind the F gate after
defeating the boss.

The Disappearing Bridge (C) might seem like a challenge but just run
across fast and you'll be fine.

You'll have to fight the second level boss, another Massacre Machine
(HP 2000) somewhere here.

You'll then have to find the correct switch sequence to get upto the
next plateau to get your hands on the Teleportation Orb - experiment
a bit and you should find it easily (I think it was something like left,
middle, top, bottom for me ...)

Head back to Dight Village and let another bit of the story unfold
(in the process of which you'll have to go back to the Mysterious Vanishing
Shrine)

Go back to the Dight Inn and talk to Gadwin and he'll want you to
visit him at home and this means a trek through the Valley of the Flying
Dragon and a fight with Gadwin to test your strength at the end of it.
So be rested and fully recovered - only Justin fights as this is a trial
of strength rather than the usual good vs. evil battle :-)

Head back to Dight, go to the pier, get on the Sea Dragon and set
sail!